If you want to create a hidden item in an Exchange folder via WebDAV, you may be tempted to set the DAV:ishidden property to true. But in order to hide the item from Outlook, you must additionally set the property PR_Associated property to true. The PR_Associated property is available via http://schemas.microsoft.com/mapi/proptag/x67aa000b.
Note that you must specify both properties during the creation of an element. Toggling the visibility of an item after it has been created is not possible.